Refrigerant system with variable capacity expander

ABSTRACT

A refrigerant system incorporates a variable capacity expander. A bypass line selectively bypasses at least a portion of the refrigerant approaching the expander to the intermediate expansion point within the expander. In this manner, the refrigerant expansion process is controlled more efficiently than in the prior art.

BACKGROUND OF THE INVENTION

This application relates to refrigerant systems, wherein the expansionprocess is provided by an expander. The capacity of the expander can bevaried by controlling the amount of refrigerant that is diverted to anintermediate expansion point in the expander. By controlling the amountof diverted refrigerant, the overall refrigerant system can be moreefficiently controlled and operated, as will be explained below.

Refrigerant systems are known in the air conditioning and refrigerationart, and are utilized to condition a secondary fluid, such as air, wateror glycol solution, that is delivered to a climate-controlled zone orspace. In a basic refrigerant system, a compressor compresses arefrigerant and delivers it downstream to a first heat exchanger, whereheat is rejected, directly or indirectly, from the refrigerant to anambient environment. From this first heat exchanger, the refrigerantpasses through an expansion process, where it is expanded to a lowerpressure and temperature, and then through a second heat exchanger,where heat is accepted by the refrigerant from a secondary fluid to coolthis secondary fluid to be delivered to an indoor environment. The firstheat exchanger is normally called a condenser, for system operationbelow the refrigerant critical point, or so-called subcriticaloperation, and is called a gas cooler, for system operation above therefrigerant critical point, or so-called supercritical operation. Thesecond heat exchanger typically operates in a subcritical two-phaseregion and is called an evaporator.

One performance enhancement option that is utilized in known refrigerantsystems is the use of an expander. As compared to restriction typeexpansion devices, whether of fixed or adjustable type, the expanderoffers advantages of a more efficient isentropic expansion process,resulting in a higher refrigerant cooling potential in the evaporator,as compared to an isenthalpic process for restriction type expansiondevices. Also, some expansion work can be recovered to assist in drivingat least one of refrigerant system components. Both expansion workrecovery and additional refrigerant cooling potential realized in theevaporator are beneficial to the refrigerant system operation, sincethey augment refrigerant system capacity and efficiency.

The use of the expanders for CO₂ refrigerant applications is especiallyimportant, as on a relative basis, the expanders provide much largerthermodynamic cycle improvements for CO₂ refrigerant than for thetraditional refrigerants. It is also important to use expanders withinthe CO₂ systems, as these systems are not as thermodynamicallyefficient, on an absolute scale, as the systems with conventionalrefrigerants, such as R22, R410A, R404A, R407C, R134a, etc.

One problem in using the expanders is related to a difficulty ofcontrolling the amount of refrigerant passing through the expander.Further, because of their transcritical nature of the CO₂ cycle, thesesystems are more sensitive to the refrigerant charge management thansystems with conventional refrigerants.

The prior art systems relied on the refrigerant bypass around theexpander (from its inlet to its outlet) to adjust the refrigerant flowthroughout the system. In other words, a portion of the refrigerant flowwas short-circuited to pass directly from the heat rejection heatexchanger outlet into the evaporator inlet. The use of this bypassproved to be inefficient, as the bypassed refrigerant represents adirect “leakage” from the expander inlet to the expander outlet, doesnot participate in the work recovery and is known to be one of the majorcontributors to the expander inefficiency.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, an expander capacity isadjusted by providing an intermediate pressure port in the expander. Ifit is desirable to pass more refrigerant through the expander, then aportion of the refrigerant flow from an expander inlet is diverted tothe intermediate expansion port. The amount of refrigerant passingthrough the expander is controlled by appropriate sizing and/or adequaterestriction placed in the bypass line. Preferably, the flow ofrefrigerant in the bypass line is controlled by a flow control devicesuch as a valve. For instance, this valve can be of an ON/OFF type, suchas a solenoid valve. The valve can also be of an adjustable restriction(modulation) type or of a pulsation type, for even more precise controlof the refrigerant flow through the bypass line. A similar technique canbe used if an expander consists of multiple expansion stages orexpanders that are installed in series with each other. In this case,some of the refrigerant is diverted from the inlet of the upstreamexpansion stage into the inlet of the expansion stage locateddownstream. In other words, in this case, the refrigerant is injectedbetween the expansion stages.

In this invention, the efficiency of the expansion process is improvedby eliminating the direct “leak” path from a high pressure heatrejection heat exchanger to a low pressure evaporator, while maintainingthe ability to provide precise control over the amount of refrigerantpassing through the expander. Furthermore, due to additional workrecovery obtained from the bypassed refrigerant and more efficientisentropic process, the refrigerant system's operational performance isimproved.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a prior art refrigerant system.

FIG. 2 shows an inventive refrigerant system.

FIG. 3 shows another schematic of an inventive refrigerant system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A prior art refrigerant system 20 is illustrated in FIG. 1. As known, acompressor 22 compresses a refrigerant and delivers it to a heatrejection heat exchanger 24, which is a condenser, for subcriticalapplications, and a gas cooler, for transcritical applications. From theheat rejection heat exchanger 24, the refrigerant, expanding to a lowerpressure and temperature, drives an expander 26. The expander 26 isshown schematically and includes a moving member that is driven by theexpanding fluid. The expansion work recovered by the expander may beutilized (directly or indirectly) to assist in driving at least one ofrefrigerant system components. In other words, the expander can beconnected to other system components, such as a compressor, a fan, or apump, either directly through a coupling, a clutch, a gearbox, etc., orcan be used to drive a generator to produce electric energy.

In order to control the expansion process through an expander, the priorart refrigerant systems have utilized a bypass line 28 that routed atleast a portion of the refrigerant from the outlet of the heat rejectionheat exchanger 24 to the inlet of the evaporator 36, at operatingconditions when the expander could not handle all of the expandingrefrigerant flow. In cases when the expander could not handle all of theexpanding refrigerant flow, the refrigerant system performance wouldhave become sub-optimized, with the refrigerant pressure in the heatrejection heat exchanger rising above a desired level and evaporatorsuperheat also potentially increasing above the desired value. An inlet32 to the bypass line 28 extends to an outlet point 33. When a bypassvalve 34 is opened, the refrigerant could flow through the bypass line28, and thus the amount of refrigerant moving through the circuit can beincreased. However, the use of this bypass is ineffective as itessentially creates a high-to-low refrigerant “leak” bypassing theexpander 26. In other words, as more refrigerant is bypassed around theexpander 26, less useful work can be recovered by the expander.Furthermore, a portion of the refrigerant that flows through the bypassvalve 34 undergoes isenthalpic expansion, which is lessthermodynamically efficient than an isentropic expansion process in theexpander 26.

The present invention is shown in FIG. 2 as a refrigerant system 50.Here, the bypass inlet 32 leads to a bypass line 52. A restriction 54can be positioned on the bypass line 52, and the point 56 terminates thebypass line 52 at an intermediate expansion point in the expander 26.The restriction 54 may be an ON/OFF, modulation or pulsation valve. Inthis invention, when at least a portion of the refrigerant bypassesthrough the valve 54, the entire refrigerant still moves through andexits the expander 26. A portion of the refrigerant that bypassesthrough the valve 54 continues to undergo an expansion process from theintermediate expansion point 56 to the expander exit point 58. In thismanner, part of the expansion work from the refrigerant passing throughthe bypass line 52 is still recovered in the expander 26, as well as, atleast partially, this bypassed portion of the refrigerant will beexpanded isentropically. At the same time, pressure upstream of theexpander 26 can be controlled by the same valve 54 to optimize theoperation of the refrigerant system 50.

The present invention increases the efficiency and capacity of arefrigerant system by including a variable capacity expander, while atthe same time, controlling the system operation to be in the optimumdomain. The present invention can be extended to an expander consistingof several expansion stages, as for example, can be a case for amulti-stage turbine. It can also be extended to a system configurationof expanders installed in series with each other. In this case, as shownin FIG. 3 for an embodiment 70, an intermediate expansion point 156 islocated between the expansion stages (or independent expanders) 26A and26B. Of course, more than two expanders can be installed in series withthe bypass line routed into the point between any stages. Further, morethan one bypass line can be installed when more than two expansionstages are connected serially.

Also, if needed, there can be multiple bypass lines 52. As shown in FIG.3 embodiment, one bypass line 52 extends through the flow control valve54 from a point 200 upstream of the first expansion stage 26A to anintermediate expansion point 202 within the same expansion stage 26A,while another bypass line 52, also incorporating the flow control valve54, extends from a point 32 upstream of the first expansion stage 26A toa point 156 intermediate of two expansion stages 26A and 26B. Obviously,upstream points 32 and 200 can be combined into a single point.

As mentioned above, the bypass valve 54 can be of a variable area typeto provide condition dependant control of how much refrigerant is routedinto the bypass line 52. The bypass valve 54 can also operate in a pulsewidth modulated manner, such that it is rapidly cycling between ON andOFF positions.

The present invention is particularly well suited for use in refrigerantsystems incorporating CO₂ as a refrigerant, where the benefits of usingan expander are the most pronounced.

It should be pointed out that many different expander and compressortypes could be used in this invention. For example, scroll, screw,rotary or reciprocating expanders and compressors can be employed.

The refrigerant systems that utilize this invention can be used in manydifferent applications, including, but not limited to, air conditioningsystems, heat pump systems, marine container units, refrigerationtruck-trailer units, and supermarket refrigeration systems.

Although a preferred embodiment of this invention has been disclosed, aworker of ordinary skill in this art would recognize that certainmodifications would come within the scope of this invention. For thatreason, the following claims should be studied to determine the truescope and content of this invention.

1. A refrigerant system comprising: a compressor, said compressor compressing a refrigerant and delivering this refrigerant to a downstream heat rejection heat exchanger, refrigerant from the heat rejection heat exchanger passing through an expander, said expander being operable to recover at least a portion of work from the refrigerant expansion process, and a bypass line for selectively bypassing at least a portion of refrigerant from a point upstream of said expander to an intermediate pressure point in said expander.
 2. The refrigerant system as set forth in claim 1, wherein said bypass line has a refrigerant flow restriction.
 3. The refrigerant system as set forth in claim 2, wherein said refrigerant flow restriction is a valve.
 4. The refrigerant system as set forth in claim 3, wherein the valve is an on/off valve.
 5. The refrigerant system as set forth in claim 4, wherein said on/off valve has capability to rapidly cycle between on and off positions.
 6. The refrigerant system as set forth in claim 5, wherein said cycle rate of said valve is between 1 second and 60 seconds.
 7. The refrigerant system as set forth in claim 2, wherein said refrigerant flow restriction has a variable restriction area.
 8. The refrigerant system as set forth in claim 1, wherein said recovered portion of work from the refrigerant expansion process is utilized to assist in driving at least one of refrigerant system components.
 9. The refrigerant system as set forth in claim 8, wherein said system component is said compressor.
 10. The refrigerant system as set forth in claim 1, wherein said expander consists of multiple expansion stages.
 11. The refrigerant system as set forth in claim 10, wherein at least one of said expansion stages is an independent expander.
 12. The refrigerant system as set forth in claim 10, wherein said bypass line bypasses the refrigerant from an upstream location of one of said expansion stages to an upstream location of a downstream one of said expansion stages.
 13. The refrigerant system as set forth in claim 10, wherein there are multiple bypass lines between said expansion stages.
 14. The refrigerant system as set forth in claim 1, wherein said refrigerant is CO₂.
 15. The refrigerant system as set forth in claim 1, wherein the recovered work is utilized to power another component by providing at least one of rotational energy and electrical power.
 16. A method of operating a refrigerant system including the steps of: (1) compressing a refrigerant and delivering this refrigerant to a downstream heat rejection heat exchanger, refrigerant from the heat rejection heat exchanger passing through an expander, said expander being operable to recover at least a portion of work from the refrigerant expansion process; and (2) selectively bypassing at least a portion of refrigerant from a point upstream of said expander to an intermediate pressure point in said expander.
 17. The method as set forth in claim 16, wherein a valve controlling the selective bypassing is rapidly cycled between on and off positions.
 18. The method as set forth in claim 17, wherein a cycle rate of said valve is between 1 second and 60 seconds.
 19. The method as set forth in claim 16, wherein said recovered portion of work from the refrigerant expansion process is utilized to assist in driving at least one of refrigerant system components.
 20. The method as set forth in claim 18, wherein said bypass line bypasses the refrigerant from an upstream location of one of said expansion stages to an upstream location of a downstream one of said expansion stages. 